Swamp coolers work by using a process called evaporative cooling. Evaporative cooling adds moisture to the air by taking warm external air through wet cooling pads, which in effect cools that air. Now both relative humidity and dew point measure how much water vapor is in the atmosphere.
Relative humidity is temperature-dependent while Dew point is not. Dew point is simply a measure of how much moisture is in the air at a given time. So when the water in the air (dew point) matches the water on the pads, then no cooling can take place.
In other words, these coolers rely on mmoisture in the air to operate efficiently. If there is too much, like there is in humid weather, then cooling is less or nonexistent, usually when the dew point is above 51% even with less humidity which is temperature driven.